The invention relates to a method of coating a substrate or support. More particularly, the invention concerns a method of coating a substrate or support with a liquid film in a manner that minimizes defects due to temperature gradients in the liquid film during coating.
When a liquid composition is coated on a support to form a film, the support is most often driven in movement along its longitudinal axis on conveyer rollers. From that moment, the layer formed on the support is subject to temperature gradients induced by the conveyor equipment or by the evaporation of the solvent of the liquid composition. In other words, temperature variations are observed from one point to another of the layer formed in the direction of conveyance. As the surface tension varies according to temperature, this gradient causes variations of surface tension and surface stresses. These surface stresses induce movements in the layer, known as the Marangoni effect. The Marangoni effect is described in xe2x80x9cPhysical Chemistry of Surfacesxe2x80x9d by Arthur W. Adamson published by John Wiley and Sons, page 122. This movement in the layer, extending from a point where the temperature is high (weak surface tension) to a point where the temperature is low (high surface tension), generates many defects (lines, stripes, agglomerates, etc.). Some surfactants enable these defects to be minimized. However, assessment of the efficiency of a surfactant to minimize these defects is not easy. It cannot be done with simple measurements of surface tension.
Therefore, it is desirable in the art to have a method of coating a support with a liquid film that includes identification and selection of a suitable surfactant that minimizes coating defects due to temperature gradients in the liquid film.
It is, therefore, an object of the present invention to provide a method for coating a support with a liquid composition comprising an aqueous or organic solvent and at least a surfactant.
Another object of the invention is to provide a method for selecting a surfactant for a coating composition comprising an aqueous or an organic solvent and at least a surfactant.
Yet another object of the invention is to provide a method for determining the optimum concentration of a surfactant to be used in a coating composition having an aqueous or an organic solvent component and at least a surfactant component to minimize coating defects due to a temperature gradient.
These objects and others are achieved by the present invention which relates to a method for coating a support with a layer of a composition comprising an aqueous or an organic solvent and at least a surfactant, wherein said surfactant agent has been selected by a sequence comprising the steps of: a) after formation of said layer, applying temperatures Ti to n successive zones Zn of said layer so that a temperature gradient xcex94T is created between a zone Zi and a zone Zi+1 and the temperatures of the zones Zi and Zi+2 are the same, wherein n is an integer representing the number of zones, i is an integer between 1 and n, and xcex94T is chosen so as to create a measurable periodic variation of thickness of said layer from the 1st to the nth zone; b) measuring said thickness Hx of each zone; c) determining an average variation of said thickness xcex94Hm of said layer; and d) repeating steps (a) to (c) for a control composition comprising said aqueous or said organic solvent to determine xcex94Hm, wherein said surfactant satisfies the following condition: xcex94Hm of said layer with said surfactant  less than xcex94Hm for said control.
In another aspect of the invention, a method for selecting a surfactant for coating a support with a layer of a composition comprising an aqueous or an organic solvent and at least a surfactant, comprising the following steps: a) coating said support with said composition; b) after formation of said layer, applying temperatures Ti to n successive zones Zn of said layer, so that a temperature gradient xcex94T is created between a zone Zi and a zone Zi+1 and said temperatures of said zones Zi and Zi+2 are the same, wherein n is an integer representing the number of zones, i is an integer between 1 and n, and xcex94T is chosen so as to create a measurable periodic variation of thickness of said layer from the 1st to the nth zone; c) measuring said thickness Hx of each zone; d) determining an average variation of said thickness xcex94Hm of said layer; and e) repeating steps (a) to (d) for a control composition comprising said aqueous or said organic solvent, to determine xcex94Hm for said control, wherein said surfactant satisfies the following condition: xcex94Hm of the layer with the surfactant  less than xcex94Hm for the control.
in yet another aspect of the invention, a method of determining optimum concentration of a surfactant in a composition used for coating a layer on a support, said composition comprising an aqueous or an organic solvent, said method comprising the steps of: a) providing m coating compositions, with m being an integer  greater than 1, each coating composition having a different concentration of said surfactant; b) coating said each coating composition on a support and one composition per support, to obtain m separate coated supports; c) after formation of each layer, applying temperatures Ti to n successive zones Z of said each layer, so that a temperature gradient xcex94T is created between a zone Zi and a zone Zi+1 and said temperatures of the zones Zi and Zi+2 are the same, n being an integer, i being an integer between 1 and n, and xcex94T being chosen so as to create a measurable periodic variation of thickness of each layer from the 1st to the nth zone; d) measuring said thickness Hx of said each zone, and e) determining average variation of said thickness xcex94Hm of said each layer, an optimum concentration of said surfactant being that of said layer which has the lowest average thickness variation xcex94Hm.
The present invention, therefore, has numerous advantageous effects over prior art developments including its ability to minimize defects in the coated support when the coating composition undergoes a temperature gradient during coating.